Surface acoustic wave transversal filters are generally constructed by spatially weighting the transducers with the desired impulse response. Several weighting schemes have been proposed. Some of the more commonly used are apodization and withdrawal weighting. Both have been successful but they each have definite limitations. Using apodization, the overlap of adjacent transducer electrodes, is proportional to the desired weighting at that position in the transducer. This causes the acoustic wave to be nonuniform across the acoustic aperture. As a result, the frequency response of the filter with two apodized transducers is not the product of the separate response functions. As a consequence, although filters utilizing apodization techniques for both input and output transducers produce predictable results, they are difficult to synthesize. Thus, in practice, SAW filters are not designed with two apodized transducers unless a multi-strip coupler is also included to make the acoustic beam uniform across its aperture. A second drawback of apodization is that regions of the transducer with small weights have very short overlaps. The narrow acoustic beam generated by this small overlap often suffers from unacceptable diffraction problems.
Withdrawal weighting solves some of the problems of small electrode overlap and nonuniform acoustic waves by having all of the transducer electrodes the same length. The weighting is obtained by removing electrodes so that the local density of the remaining electrodes is approximately equal to the desired weighting in that region. Even though withdrawal weighting results in a full acoustic aperture, it works by averaging the effects of a group of electrodes over a region and is thus inherently limited by the fact that the desired impulse weighting cannot always be adequately approximated by a withdrawal pattern.
Another technique for producing weighting in a transducer is the series sections weighting. It works by breaking down an individual electrode in several components all connected effectively in series. An electrode pair is fabricated by a specified number of radiators which are electrically coupled to each other in series. Effectively, each electrode pair results in a specified number of capacitors all connected in series that divide the voltage across that electrode and hence the weighting of that electrode pair is the reciprocal of the number of series sections. One problem with series section weighting is that the weights are restricted to reciprocals of integers and, therefore, there is an insufficient control over large weighting values. For very small weights, the number of series sections needed becomes very large and the fraction of aperture taken up by the transition between series sections becomes significant and the corresponding error due to end effects at each transition may become large.
All of the weighting techniques for transducers have disadvantages that result in compromised performance of SAW devices.